Chlorophyll Pigment & SST Cycles

Physical-Biological Modelling

The Gulf of Mexico has been the subject of many physical oceanographic field and modelling studies. Its Loop Current and shedding of large anticylconic rings are often observed. Less common, however, are studies that concentrate on the biological nature of the Gulf. Monthly climatologies of the sea-surface temperature (SST) and chlorophyll pigment concentrations were used by Walsh et al. (1989) to validate a coupled physical-biological model for the Gulf of Mexico, specifically to model the seasonal biological response to eddy shedding and nutrient injection by the Loop Current, incident light, and vertical mixing.

These data were also used by Muller-Karger et al. (1991) in order to examine the spatial and temporal variability of the surface distribution of phytoplankton.

Pigment Concentrations:

The Coastal Zone Color Scanner determines the water-leaving radiance at 6 visible and IR bands. Concentrations are derived using ratios of the blue (443 nm) or blue-green (520 nm) to the green radiance (550 nm) values. Values are obtained using atmospheric correction and bio-optical (derived empirically) algorithms valid for deep, low-chlorophyll waters (0.08-1.5 mg/m³). Satellite values provide an estimate of the average pigment concentration in the first water optical depth, which at low concentrations (0.04-0.5 mg/m³) represent a layer of 10-25 m in depth. These pigment values have been validated and found to agree within 40% under optimal conditions. Case II waters possess large concentrations of colored dissolved organic matter (Gelbstoff), marine or terrigenous effluent from rivers, or resuspended sediments. Satellite-derived values for pigment concentrations in Case II waters are not as accurate as Case I since phytoplankton do not covary with these other constituents, making the CZCS blue-green ratio algorithm unfeasible. Bio-optical algorithms often overestimate the chlorophyll pigment concentration by attributing the strong signal to pigment when it is due to the presence of other material. As such, the accuracy of retrieved chlorophyll values are questionable over shallow shelf waters and regions of river plume dispersal.

Sea-surface Temperature:

Monthly SST images have also been created by Muller-Karger et al. (1991). The global monthly average bias error is less than 0.1° C, and the root mean square (rms) error is less than 0.8° C. Individual points, however, may be less accurate. The multichannel sea-surface temperature (MCSST) procedure is described in great detail in McClain et al. (1990). Monthly profiles of temperature, salinity, and depth were also created from NOAA National Oceanographic Data Center (NODC) station data.

Sampling Notes:

The entire pigment time series extends over the life of the CZCS, from November 1978 to November 1985. Due to scheduling, satellite position, and satellite power drains, 41% of the data examined were collected in just 1979 and the first six months of 1980. Temporal sampling for June 1980 through December 1982 resulted in markedly decreased coverage. Coverage is completely lacking in November 1980, June 1981, April 1984, and May 1985.

Cloud cover also aliases time series of pigment fields, since periods of atmospheric frontal passage or storms are far more prevalent during certainyearly periods.

Chlorophyll Characteristics:

The CZCS images of monthly mean pigment fields in the Gulf of Mexico from November 1978 through November 1985 have been derived. (1979, 1980, 1981, 1982, 1983, 1984, 1985). The CZCS time series of phytoplankton concentration show that seasonal variation in pigment concentration seaward of the shelf is synchronous throughout the Gulf, with highest values (>0.18 mg/m³) in December-February, and lowest values (~0.06 mg/m³) in May-July. This synchronicity may be more clearly seen in a time series of two 200-km square subregions of the Gulf — one in an offshore area under Loop Current influence, and the other without (East vs. West). The algal biomass cycles are clearly similar, showing that temporal pigment variability in offshore waters is independent of the Loop Current or the presence of anticyclonic eddies.

It is important to note these time series are robust only up to about 1982; beyond this, artifacts may have been introduced by the drastic decrease in CZCS sampling, such as volcanic eruptions (Mt. Chichon) and sensor instability. The climatologies derived here include this part of the record as well.

Plume and Loop Current Recognition:

When local processes of wind mixing do not dominate the surface chlorophyll field, CZCS imagery may be used to delineate circulation features of the oligotrophic state of the Gulf of Mexico. For example, the more robust portion of the time series of pigment images (1979, 1980) show that during summer there was marked spatial structure of low algal biomass associated with the Loop Current and anticyclonic eddies. The eastern Gulf was dominated by the clear-water intrusion of the summer Loop Current, while the western side contained patches of clear water associated with anticyclonic rings of downwelling cores, where nutrient depletion of shallow mixed layers is accentuated during summer periods.

Pigment field structure disappears in winter, however, as concentrations increase simultaneously throughout the Gulf. Offshore pigment fields become horizontally homogeneous as early as November and do not develop significant spatial structure again until about February of the following year.

The ocean color images show that most of the water discharged by the Mississippi River flows to the west, following the Louisiana-Texas coast, at times reaching south of the Mexico $ndash; United States border. For example, the CZCS composites for 1979 and 1980 show that most of the Mississippi River discharge was carried west in a band following the coast and extending at least as far as Tampico, Mexico. There were no apparent seasonal changes in the direction of the dispersal of the plume. However, there were large interannual differences in the size of the plume (length and width) consistent with variations in volume discharge.

Biological Productivity:

The biological productivity of the shelf is strongly affected by Mississippi River effluent, outflow from coastal lagoons and smaller rivers, cyclonic eddies which develop along the continental margin, and wind-driven upwelling through the mixed layer (Walsh, 1988). A dearth of nutrient and primary productivity observations for the Gulf of Mexico make estimation of the relative contribution of nutrient supply mechanisms difficult. The following points may be noted to form a picture of the processes leading to the observed chlorophyll pigment patterns:

During a quasi-annual penetration and eddy-shedding cycle of the Loop Current, the simulated seasonal changes of incident light, winds, and surface mixed layer depth (from Levitus, 1982) induce an annual cycle of algal biomass that correspond well to in-situ and satellite time series of chlorophyll. The modelled phytoplankton concentrations were highest when the mixed layer was deepest (100 m) and reached a minimum simultaneously with the shallowest mixed layer (25 m), such that flux minima occur during periods of greatest water stratification.

Surface wind stress dictates the depth of the model's surface mixed layer: both are greatest in the winter months (November to March).

Maximal chlorophyll concentrations are found from November to January beyond the shelf break.

Maximum northward penetration of the Loop Current occurs in March, April, and May (Vukovich 1979).

Riverine input is often high in chlorophyll, nutrients, and dissolved organic matter; Mississippi outflow is greatest during spring (January-March).

Thus, as winds increase and air temperature drops in early winter, the surface mixed layer deepens due to heat loss and convective overturn, increasing the nutrient supply and pool. Enhanced autumn river discharge also aids in phytoplankton growth and supply to the Gulf waters. During the winter, phytoplankton growth flourishes with maximal nutrient availability that is further enhanced by the northward penetration in early spring by the Loop Current, promoting upwelling in the surrounding waters and nutrient supply from deep water. Growth is also enhanced by the increased spring river outflow that is high in chlorophyll and nutrients. Then, as spring moves to summer, the Loop Current retreats, cutting off nutrient supply from upwelling, river outflow decreases, and winds die, shallowing the surface mixed layer. All these factors lead to a decreased nutrient supply and pool, inhibiting phytoplankton growth. These reasons combine to explain the lower phytoplankton concentrations found during the summer months.

Weekly SST Composites:

Weekly SST composites have been created from AVHRR images from late 1993 through the first half of 1996. The SST changes were synchronous between the eastern and western portions of the Gulf. Spatial SST structure in the AVHRR images are poorly developed during summer (May through October) but very well developed from November through May. Thus, infrared satellite images provide synoptic maps of the Loop Current and its eddies only during a 7-month period of the year. The remainder of the year sees an insufficient temperature gradient that makes identifying the Loop Current from IR imagery impossible.

SST and Pigment Concentration Complementarity:

The combined use of ocean color and infrared images permits year-round observation of spatial structure of the surface circulation in the Gulf. Thermal infrared images are most useful between November and mid-May, when strong temperature gradients occur. The SST fields are uniform between late May and October, but the Loop Current, large anticyclonic eddies, and Mississippi River plume may be traced with chlorophyll pigment concentrations using ocean color sensors.

Despite many oceanographic studies of the Gulf of Mexico, questions of the processes controlling ring and eddy formation or frequency of eddy shedding, for example, remain unanswered. Satellite observations on a year-round basis are now possible with the upcoming launches of the Japanese Ocean Color Temperature Sensor (OCTS) and SeaWiFS. These observations provide an unparalleled glimpse of the processes governing the Gulf of Mexico.